The present disclosure relates generally to data transfer on an industrial process network. More particularly, the present disclosure relates to a fast data transfer communication protocol between devices on the industrial process network.
In a typical industrial plant, a distributed control system (DCS) is used to control many of the industrial processes performed at the plant. Typically, the plant has a centralized control room having a computer system with user input/output (I/O), disc I/O, and other peripherals. Coupled to the computer system are a controller and a process I/O subsystem. The process I/O subsystem includes I/O ports, which are connected to various field devices throughout the plant. Field devices include various types of analytical equipment, such as pressure sensors, temperature sensors, switches, transducers, valve positioners and actuators, as well as any other device that performs a function in a distributed control system.
Traditionally, analog field devices have been connected to the control room by two-wire twisted pair current loops, with each device connected to the control room by a single two-wire twisted pair. Analog field devices are capable of responding to or transmitting an electrical signal within a specified range. In a typical configuration, it is common to have a voltage differential of approximately 20-25 volts between the two wires of the pair and a current of 4-20 milliamps running through the loop. An analog field device that transmits a signal to the control room modulates the current running through the current loop, with the current proportional to a sensed process variable. On the other hand, an analog field device that performs an action under control of the control room is controlled by the magnitude of the current through the loops, which is modulated by the I/O port of the process I/O system, which in turn is controlled by the controller.
Historically, most traditional field devices have had either a single input or a single output that was directly related to the primary function performed by the field device. For example, often the only function implemented by a traditional analog resistive temperature sensor is to transmit a temperature by modulating the current flowing through the two-wire twisted pair, while the only function implemented by a traditional analog valve positioner is to position a valve between an open and closed position based on the magnitude of the current flowing through the two-wire twisted pair.
More recently, hybrid systems that superimpose digital data on the current loop have been used in process control systems. One hybrid system is known in the control art as the Highway Addressable Remote Transducer (HART) and is similar to the Bell 202 modem specification. The HART system uses the magnitude of the current in the current loop to sense a process variable (as in the traditional system), but also superimposes a digital carrier signal upon the current loop signal. The carrier signal is relatively slow, and can provide updates of a secondary process variable at a rate of approximately 2-3 updates per second. Generally, the digital carrier signal is used to send secondary and diagnostic information and is not used to realize the primary control function of the field device. Examples of information provided over the carrier signal include secondary process variables, diagnostic information (including sensor diagnostics, device diagnostics, wiring diagnostics, and process diagnostics), operating temperatures, temperature of the sensor, calibration information, device identification information, materials of construction, configuration or programming information, or other types of information. Accordingly, a single hybrid field device may have a variety of input and output variables and may implement a variety of functions.
Foundation Fieldbus is a multi-drop serial digital two-way communications protocol defined by the Instrument Society of America (ISA), and is intended for connecting field instruments and other process devices (e.g., monitoring and simulation units) in distributed control systems. Foundation Fieldbus allows enhanced digital communication over previous process control loop methods while maintaining the ability to power process devices coupled to the Fieldbus loop and while meeting intrinsic safety requirements. For instance, the Foundation Fieldbus specification (i.e., including the physical layer specification and the data link layer specification) defines networks that transmit data at much higher data rates than traditional hybrid systems, such as at data rates up to 31.25 kilobits per second (Kbps) for an H1 Fieldbus network and data rates up to 2.5 megabits per second (Mbps) for an H2 Fieldbus network.
The Fieldbus Message Specification (FMS) defines a messaging protocol for communications over the fieldbus, accomplished via Virtual Communication Relationships (VCRs). The VCRs provide connection-based channels for the transfer of data between applications and/or devices. Devices on the fieldbus network communicate via both scheduled and unscheduled communications managed by a Link Master (LM) device that is designated as the Link Active Scheduler (LAS). During scheduled communications, the LAS device issues a compel data message to a field device. In response, the field device publishes data over the network to one or more subscriber devices. Unscheduled communications are accomplished via a token passing algorithm managed by the LAS device. The LAS device issues a pass token message in turn to each device included in a list of active devices on the network, often referred to as the live list. Upon receiving the pass token message, a field device transmits any unscheduled data until all of its data has been published or a configurable “maximum token hold time” has expired.
Due to the connection-based nature of the VCRs, data transmissions are accomplished via a series of messages, each requiring a corresponding acknowledgment. Unacknowledged messages are retransmitted, thereby increasing robustness of communications. However, such connection-based transactions introduce overhead to the communications scheme, which is time-limited by the maximum token hold time allotted to each device for unscheduled communications. Accordingly, transactions involving large amounts of data, such as uploads and downloads of configuration data and/or software images can be relatively slow, sometimes taking an hour or more to complete the transaction. As such, the use of connection-based transactions can increase the time (and expense) associated with activities such as maintenance, debugging, and device commissioning, and thereby generally decreasing usability of the system.